1. Field
An aspect of the present disclosure relates generally to a semiconductor memory device and an operating method thereof.
2. Description of the Related Art
Semiconductor memory devices are generally classified into volatile memory devices and nonvolatile memory devices.
A non-volatile memory device performs a read/write operation at a relatively lower speed than a volatile memory device, but, unlike a volatile memory device, retains stored data even when power supply to the device is cut off. Accordingly, a non-volatile memory device is used to store data to be retained regardless of a power supply. Examples of a non-volatile memory device are a read-only memory (ROM), a mask ROM (MROM), a programmable ROM (PROM), an erasable programmable ROM (EPROM), an electrically erasable programmable ROM (EEPROM), a flash memory, a phase-change random access memory (PRAM), a magnetic RAM (MRAM), a resistive RAM (RRAM), a ferroelectric RAM (FRAM), and the like. Flash memories are widely used and may be classified into NOR and NAND flash memories.
The flash memories have an advantage of a RAM in which data is freely programmed and erased, and an advantage of the ROM in which stored data is retained even when a power supply is cut off. The flash memories are widely used as a storage medium of portable electronic devices, such as a digital camera, a personal digital assistant (PDA), and an MP3 player.
Program characteristics of a plurality of memory cells included in the flash memory may be different from each other. For example, since magnitudes of threshold voltages increasing according to the same program voltage are different from each other, fast cells programmed at fast speed and slow cells programmed at slow speed may exist in a program operation of the flash memory. The width of a threshold voltage distribution width of a flash memory is therefore widened by the presence of fast and slow cells.